Ubiquitination is an enzymatic post-translational modification that affects protein fate. The ubiquitin-proteasome system (UPS) was first discovered in reticulocytes where it plays important roles in reticulocyte maturation. Recent studies have revealed that ubiquitination is a dynamic and reversible process and that deubiquitylases are capable of removing ubiquitin from their protein substrates. Given the fact that the UPS is highly active in reticulocytes, it is speculated that deubiquitylases may play important roles in erythropoiesis. Yet, the role of deubiquitylases in erythropoiesis remains largely unexplored. In the present study, we found that the expression of deubiquitylase USP7 is significantly increased during human terminal erythroid differentiation. We further showed that interfering with USP7 function, either by short hairpin RNA-mediated knockdown or USP7-specific inhibitors, impaired human terminal erythroid differentiation due to decreased GATA1 level and that restoration of GATA1 levels rescued the differentiation defect. Mechanistically, USP7 deficiency led to a decreased GATA1 protein level that could be reversed by proteasome inhibitors. Furthermore, USP7 interacts directly with GATA1 and catalyzes the removal of K48-linked poly ubiquitylation chains conjugated onto GATA1, thereby stabilizing GATA1 protein. Collectively, our findings have identified an important role of a deubiquitylase in human terminal erythroid differentiation by stabilizing GATA1, the master regulator of erythropoiesis.

Iron is required for hemoglobin production, and it plays a key role during erythropoiesis. Systemic iron homeostasis is mainly negatively regulated by the peptide hormone hepcidin, coded by the gene HAMP. Hepcidin excess may cause iron deficiency, iron-restricted erythropoiesis, and anemia. Conversely, hepcidin insufficiency leads to iron overload and oxidative damage in multiple tissues. During regulation of hepcidin synthesis, multiple promoter elements in the HAMP gene respond to variable signaling pathways corresponding to different extracellular situations. It has been reported that hepcidin expression can be suppressed by secreted erythroid factors, including GDF15, TWSG1, GDF11, and ERFE, thereby increasing iron availability for hemoglobin synthesis. These potential erythroid factors act via intricate mechanisms that remain controversial. However, it is clear that hepcidin affects erythropoiesis, and promising therapies targeting hepcidin have been developed to treat erythroid disorders. These therapeutic strategies include suppressing or activating HAMP gene expression, mimicking or activating hepcidin activity, and blocking the ability of hepcidin to bind to its target ferroportin.

Hepcidin (HAMP) synthesis is suppressed by erythropoiesis to increase iron availability for red blood cell production. This effect is thought to result from factors secreted by erythroid precursors. Growth differentiation factor 11 (GDF11) expression was recently shown to increase in erythroid cells of b-thalassaemia, and decrease with improvement in anaemia. Whether GDF11 regulates hepatic HAMP production has never been experimentally studied. Here, we explore GDF11 function during erythropoiesistriggered HAMP suppression. Our results confirm that exogenous erythropoietin significantly increases Gdf11 as well as Erfe (erythroferrone) expression, and Gdf11 is also increased, albeit at a lower degree than Erfe, in phlebotomized wild type and b-thalassaemic mice. GDF11 is expressed predominantly in erythroid burst forming unit-and erythroid colony-forming unit-cells during erythropoiesis. Exogeneous GDF11 administration results in HAMP suppression in vivo and in vitro. Furthermore, exogenous GDF11 decreases BMP-SMAD signalling, enhances SMAD ubiquitin regulatory factor 1 (SMURF1) expression and induces ERK1/2 (MAPK3/1) signalling. ERK1/2 signalling activation is required for GDF11 or SMURF1mediated suppression in BMP-SMAD signalling and HAMP expression. This research newly characterizes GDF11 in erythropoiesis-mediated HAMP suppression, in addition to ERFE.

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